Have you checked his teeth? When our hammy was two he was losing weight and not eating properly and it was because his teeth had got too long (presumably avoiding hard food due to toothache or something). I had his teeth trimmed and he went back to eating normally for quite some time.
There are lots of baby foods recommended in the thread Cypher linked I think. I went through them all and worked out which ones didn't have tomato, garlic and spices
It was this thread with all the baby food ingredients/brands in - I guess some may have changed.
Baby food suitable for hamsters
Fruity chicken and chicken and pumpkin were the ones that went down best.